A "small" comet impact on Earth is the most likely extraterrestrial object to kill millions of people and it could happen at any moment, according to a British expert.

Dr Matt Genge, from London's Natural History Museum, has investigated the damage a wide range of "small" meteors and comets would have if they struck our planet.

He identifies comets between just 50 to 100 metres wide as the most terrifyingly destructive, with massive heat and shock waves burning people and crushing buildings.

Dr Genge explains that whilst meteors bigger than two kilometres could wipe out humanity, these hits are expected only once every million years or so.

But smaller ones could still lead to the deaths of tens of millions and arrive much more frequently. These are therefore much more dangerous, he said.

Dr Genge was explaining his ideas at the British Association's Festival of Science in Sheffield, UK.

Shock wave

A 60-metre-wide comet exploded over Siberia in 1908 with 600 times the energy of the bomb dropped on Hiroshima. It laid waste to a 40-km-wide patch of forest, but, fortunately, the area was unpopulated.

Some meteorite material survives the impact

However, comets of this size are expected to strike the Earth every 100 to 300 years.

If the 1908 comet had arrived just eight hours earlier, it would have struck London, killing everything as Dr Genge described.

The comet, made of ice and dust, would hit the atmosphere at 58,000 km/h (36,000mph). As it plunged downwards, air friction would heat the object into a fireball and start to fragment it. Sonic booms would thunder from the sky, before the comet exploded with tremendous force.

Dr Genge said: "You get a large shock wave and thermal flash. It's almost
exactly the same as a nuclear air burst.

"Let's say you're a very fast thinker. In the micro-seconds you have left,
the first thing is that everything would burst into flames, including you.

"You'd be knocked off your feet by the shock wave and then dragged back again towards the explosion as all the air rushes back in."

Danger signs

After the blast, London would be a wasteland of flattened, charred buildings
and blackened corpses.

Dr Genge looked at the physical properties of meteorites and comets to identify the Siberian-type strike as the most dangerous.

Strikes from really big space rocks are rare

Ironically, it is because the 50 to 100-metre-wide comets are so weak that they are so dangerous. They break up into fragments which explode just a few kilometres above the ground - "the optimum altitude for maximum devastation", said Dr Genge.

Stone and metal meteors of similar size are much stronger and do not break up and explode.

However, 100-metre-wide metal meteors will create a blast area of 60 km across compared to just two kilometres for a stone meteor of the same size. The metal's higher density is to blame.

Dr Genge believes that this type of analysis will help to decide what level of danger an Earth-bound object would pose and aid decisions about what course of action to take.

The trajectory of most objects in the Solar System can be predicted for hundreds of years, but there still some that come with little warning, such as comet Hale-Bopp.